Optics

Incoming light is first collected by the large primary mirror at the bottom of the telescope. It is then reflected up to the secondary mirror, back down to the tertiary mirror, and then off to one of several instruments that are mounted radially at different focal stations around the side of the telescope. Only the primary and secondary mirrors are powered (curved); the tertiary serves to redirect the light to an instrument, and is essential to allow quick changes of instrument. This arrangement allows SOAR to have many instruments mounted simultaneously, without blocking the path of the incoming light.

About the Mirrors

The Primary Mirror

The primary mirror blank was made from Corning Ultra-Low Expansion glass by first making smaller hexagonal segments, then fusing them together into the larger blank. This resulted in a flat piece of glass 4.2 m in diameter but only 4 inches thick! The blank was then formed into a meniscus by heating it in a large oven and allowing it to slump over a mold. Polishing of the primary mirror was done at Goodrich Corporation (formerly Raytheon) optics facility in Danbury CT.

Active Optics Support Structure

Goodrich supplied the primary, secondary and tertiary mirrors, in addition to their active optics system, as an integrated system. The secondary mirror is mounted on a hexapod to provide 6-axis motion for low-speed active optics control.

The flat tertiary mirror provides 50 Hz tip-tilt correction, and also rotates to select the desired instrument port. It sits on a stalk that sticks up through the hole in the center of the primary mirror.